Poly(.alpha.-methyl styrene-co-acrylic acid-co-styrene) and poly(styrene-co-acrylic acid-co-methacvrylic acid), acrylic resins, are used as a polymeric surfactant in emulsion polymerizations, as a pigment grinding resin and for preparing dispersions used to make overprint varnishes. In use, the resins are suspended in water and made into a dispersion, also known as a latex, by neutralizing them with a base such as 28% ammonium hydroxide. The base allows the acrylic resin to form polymeric surfactant micelles which have two chief advantages over solvent based systems. Firstly, they have lower viscosity, which is especially evident in high-solids systems. More importantly, however, is that being substantially solvent free, they are more environmentally friendly than solvent-based systems.
Typically, the acrylic resin has been made by bulk polymerization in a continuous-stirred tank reactor (CSTR). The CSTR is charged with styrene or styrene plus .alpha.-methyl styrene, (meth)acrylic acid, a polymerization initiator and a solvent or just with styrene, .alpha.-methyl styrene and (meth)acrylic acid. Reaction temperatures range from 180.degree. C. to 300.degree. C. and residence times are from 1 to 60 minutes. Of course, level control is very important. However, pressure is not controlled. The once-through percent conversion is on the order of 75%. The acrylic resin/unreacted monomer reaction product is sent to a devolatilizer for stripping of unreacted monomers for reuse. What emerges from the devolatilizer is the desired acrylic resin, suitable for flaking, pelletizing, pulverization, etc.
Heretofore, it has been believed that the reaction pressure appears to have no significant effect on the yield, and hence, pressure has not been controlled. Also, the use of tubular reactors for the bulk polymerization of styrenics has been taught away from because of problems encountered in thermal runaway reactions at 297.degree. C., which resulted in resins having unacceptably large polydispersion. Past suggestions for avoiding this problem include the use of CSTRs with installed internal cooling coils.
The continuous tube reactor (CTR), also known as the linear flow reactor, has seen wide use in polymerizations because of its simplicity. No level controls are required, and because there is no stirring, there is no need for expensive, rotating seals capable of withstanding the pressure, temperature and solvent effects of the reaction. In the case of acrylics, it has been used in suspension polymerizations; the monomers employed are usually water soluble.
Note that all quantities appearing hereinafter, except in the examples are to be understood as being modified by the term "about." Also, all percentages are weight percentages unless indicated otherwise.